Mechanics of Luminosity . 157 



are arranged, as well as on account of their application to the 

 (iron) lines of the solar spectrum. 



Differences in the Mode of Evolution of Light, and of Emis- 

 sion of Light. Continuous and Discontinuous Excitation. 

 Store of Luminous Energy. 



11. In all investigations on luminosity two chief classes of 

 phenomena are sharply to be distinguished: first, those where 

 it is always the same particles which emit the light ; and, 

 secondly, those where continually new molecules take up the 

 luminous motions. 



The first is the case in the usual phenomena of luminosity, 

 fluorescence, electro-luminescence, &c. ; the second case occurs 

 when the luminosity is produced by chemical changes, e. g. 

 combustion, oxidation of phosphorus, of lophine, crystallization 

 of arsenious acid, and, as I shall show further on, also in the 

 luminous phenomena of phosphorescent calcium-sulphide 

 compounds. 



In the present treatise only the phenomena of the first class 

 will be treated at length. 



12. In considering the mechanics of luminosity, we must 

 observe that there are two factors to be considered together. 



First, a definite amount of energy must be communicated 

 to the molecules, which gives rise to the production of the 

 luminous motions ; and, secondly, in consequence of the 

 radiation of light a continuous diminution of this energy is 

 brought about. The final condition of the body, as far as the 

 phenomena of light are concerned, depends on the relation 

 between these two quantities. A stationary condition ensues 

 when the supply of energy is equal to the loss of energy. 



(a) The addition of energy producing light may either be 

 continuous, or it may be repeated after longer or shorter 

 intervals. 



A continuous addition occurs when a body is brought to 

 photo-luminescence by means of incident light. The interrup- 

 tions observed in the phosphoroscope are of course not to be 

 taken into account, since in comparison with the vibration- 

 period of light they are infinitely long. To the same category 

 probably belong electro- and kathode-luminescence ; since in 

 the anode light the changes in the dielectric polarization of the 

 molecules produce vibrations, whilst the kathode-rays are 

 probably connected with the ultra-violet rays*. 



A discontinuous excitation occurs in many other cases, as 

 in glowing gases. In the contact of two molecules a part of 

 the energy of translation is converted into luminous energy, 

 * Wied. Ann. xx. p. 781 (1883). 



